The positive electrode and the negative electrode of non-aqueous electrolyte secondary batteries include a compound capable of absorbing and desorbing lithium ion in the process of charge/discharge, and have high energy density. In recent years, among non-aqueous electrolyte secondary batteries, the sale of lithium ion batteries is increasing, and improvements thereof also have been continued vigorously.
An electrolyte in which a solute is dissolved in a non-aqueous solvent (organic solvent) is used as the electrolyte used for non-aqueous electrolyte secondary batteries. For example, organic solvents such as a cyclic carbonate (cyclic carbonic acid ester), a chain carbonate (chain carbonic acid ester) and a cyclic carboxylic acid ester are used as the non-aqueous solvent.
However, the non-aqueous electrolyte containing an organic solvent as its main component has inflammability. Therefore, the challenge has been the improvement of the battery safety. For example, a protective circuit for inhibiting the overcharge or overdischarge of the battery is used in order to ensure the safety.
In order to avoid the above-described problem of inflammability of the electrolyte, it has also been proposed to use a room temperature molten salt as the solvent of the electrolyte. A room temperature molten salt is a liquid at an ordinary temperature, has a very low vapor pressure, and tends not to burn. For example, an electrolyte that includes 1-methyl-3-ethylimidazolium tetrafluoroborate (MEI.BF4) as the room temperature molten salt and LiBF4 as the solute has been proposed. This electrolyte further includes ethylene carbonate (EC), which is a chain carbonate, or dimethyl carbonate (DMC), which is a cyclic carbonate, to the extent that the nonflammability is not impaired (see Patent Document 1). Further, an electrolyte in which a cyclic carbonate having a C═C unsaturated bond is mixed with MEI.BF4 has also been disclosed (see Patent Document 2).
The reason for mixing a cyclic or chain carbonate, or a cyclic carbonate having a C═C unsaturated bond with the room temperature molten salt serving as the solvent lies in the suppression of the decomposition of the room temperature molten salt. For example, MEI.BF4 tends to be reductively decomposed in a negative electrode having a potential of not more than 1 V with respect to the redox potential of lithium. Such a decomposition of the room temperature molten salt can be suppressed with a film formed on the negative electrode by a cyclic or chain carbonate, or a cyclic carbonate having a C═C unsaturated bond.
In addition, the room temperature molten salt has high viscosity. Therefore, improving the performance at high load of the battery by reducing the viscosity of the electrolyte is another reason for mixing the organic solvent with the room temperature molten salt.
Patent Document 1 Japanese Laid-Open Patent Publication No. Hei 11-260400
Patent Document 2 Japanese Laid-Open Patent Publication No. 2002-373704